The present invention relates to a controlled release drug delivery system. In particular, the present invention relates to a controlled release neurotoxin delivery system.
A controlled release system can deliver a drug in vivo at a predetermined rate over a specific time period. Generally, release rates are determined by the design of the system, and can be largely independent of environmental conditions such as pH. Controlled release systems which can deliver a drug over a period of several years are known. Contrarily, sustained release systems typically deliver drug in 24 hours or less and environmental factors can influence the release rate. Thus, the release rate of a drug from an implanted controlled release system (an "implant") is a function of the physiochemical properties of the carrier implant material and of the drug itself. Typically, the implant is made of an inert material which elicits little or no host response.
A controlled release system can be comprised of a drug with a biological activity incorporated into a carrier. The carrier can be a polymer or a bioceramic material. The controlled release system can be injected, inserted or implanted into a selected location of a patient's body and reside therein for a prolonged period during which the drug is released by the implant in a manner and at a concentration which provides a desired therapeutic efficacy.
Polymeric materials can release drugs due to diffusion, chemical reaction or solvent activation, as well as upon influence by magnetic, ultrasound or temperature change factors. Diffusion can be from a reservoir or matrix. Chemical control can be due to polymer degradation or cleavage of the drug from the polymer. Solvent activation can involve swelling of the polymer or an osmotic effect. See e.g. Science 249;1527-1533:1990.
A membrane or reservoir implant depends upon the diffusion of a bioactive agent across a polymer membrane. A matrix implant is comprised of a polymeric matrix in which the bioactive agent is uniformly distributed. Swelling-controlled release systems are usually based on hydrophilic, glassy polymers which undergo swelling in the presence of biological fluids or in the presence of certain environmental stimuli.
Preferably, the implant material used is substantially non-toxic, non-carcinogenic, and non-immunogenic. Suitable implant materials include polymers, such as poly(2-hydroxy ethyl methacrylate) (p-HEMA), poly(N-vinyl pyrrolidone) (p-NVP)+, poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), polydimethyl siloxanes (PDMS), ethylene-vinyl acetate (EVAC) copolymers, polyvinylpyrrolidone/methylacrylate copolymers, polymethylmethacrylate (PMMA), poly(lactic acid) (PLA), poly(glycolic acid) (PGA), polyanhydrides, poly(ortho esters), collagen and cellulosic derivatives and bioceramics, such as hydroxyapatite (HPA), tricalcium phosphate (TCP), and aliminocalcium phosphate (ALCAP). Lactic acid, glycolic acid and collagen can be used to make biodegradable implants.
Controlled release systems comprising a polymer for prolonged delivery of a therapeutic drug are known. For example, a subdermal reservoir implant comprised of a nonbiodegradable polymer can be used to release a contraceptive steroid, such as progestin, in amounts of 25-30 mg/day for up to sixty months (i.e. the Norplant.RTM. implant). Additionally, Dextran (molecular weight about 2 million) has been released from implant polymers.
An implant material can be biodegradable or bioerodible. An advantage of a bioerodible implant is that is does not need to be removed from the patient. A bioerodible implant can be based upon either a membrane or matrix release of the bioactive substance. Biodegradable microspheres prepared from PLA-PGA are known for subcutaneous or intramuscular administration.
A degradable implant preferably retains its structural integrity throughout its duration of controlled release so that it can be removed if removal is desired or warranted. After the incorporated drug falls below a therapeutic level, a biodegradable implant can degrade completely without retaining any drug which can be released at low levels over a further period. Subdermal implants and injectable microspheres made of degradable materials, such as lactic acid-glycolic acid copolymers, polycaprolactones and cholesterol, for steroid delivery, are known.
Protein Implants
Controlled release systems for large macromolecules, such as proteins are known. Thus, biocompatible, polymeric pellets which incorporate a high molecular weight protein have been implanted and shown to exhibit continuous release of the protein for periods exceeding 100 days. Various labile, high molecular weight enzymes (such as alkaline phosphatase, molecular weight 88 kD and catalase, molecular weight 250 kD) have been incorporated into biocompatible, polymeric implants with long term, continuous release characteristics. Generally an increase in the polymer concentration in the casting solution decreases the initial rate at which protein is released from the implant. Nature 263; 797-800:1976.
Additionally, albumin can be released from an EVAc implant and polylysine can be released from collagen based microspheres. Mallapragada S. K. et al, at page 431 of chapter 27 in Von Recum, A. F. Handbook of Biomaterials Evaluation, second edition, Taylor & Francis (1999). Additionally, the release of tetanus toxoid from microspheres has been studied. Ibid at 432. Sintered EVAc copolymer inserted subcutaneously has been shown to release insulin over a period of 100 days. Ibid at 433.
Furthermore, it is known to encapsulate a protein, such as human growth hormone (hGH) (molecular weight about 26 kD), within a polymeric matrix which when implanted permits the human growth hormone to be released in vivo over a period of about a week. U.S. Pat. No. 5,667,808.
A controlled release system (i.e. an "implant") can exhibit a high initial burst of protein release, followed by minimal release thereafter. Unfortunately, due to the high concentration of protein within a controlled release matrix, the protein molecules tend to aggregate and form denatured, immunogenic concentrations of protein.
Pulsatile Release Implants
Hydrogels have been used to construct single pulse and multiple pulse drug delivery implants. A single pulse implant can be osmotically controlled or melting controlled. Doelker E., Cellulose Derivatives, Adv Polym Sci 107; 199-265:1993. It is known that multiple pulses of certain substances from an implant can be achieved in response to an environmental change in a parameter such as temperature (Mater Res Soc Symp Proc, 331 ;211-216:1994; J. Contr Rel 15;141-152:1991), pH (Mater Res Soc Symp Proc, 331;199-204:1994), ionic strength (React Polym, 25;127-137:1995), magnetic fields (J. Biomed Mater Res, 21;1367-1373:1987) or ultrasound.
Unfortunately, a subcutaneous implantable drug pellet made of a nonbiodegradable polymer has the drawback of requiring both surgical implantation and removal. Use of a biocompatible, bioerodible implant can overcome the evident deficiencies of nonbiodegradable implants. A biodegradable implant can release a drug over a long period of time with simultaneous or subsequent degradation of the polymer within the tissue into constituents, thereby avoiding any need to remove the implant. See e.g. Drug Development and Industrial Pharmacy 24(12);1129-1138:1998.
A degradable polymer can be a surface eroding polymer, as opposed to a polymer which displays bulk or homogenous. A surface eroding polymer degrades only from its exterior surface, and drug release is therefore proportional to the polymer erosion rate. A suitable such polymer can be a polyanhydride.
Botulinum Toxin
The anaerobic, gram positive bacterium Clostridium botulinum produces a potent polypeptide neurotoxin, botulinum toxin, which causes a neuroparalytic illness in humans and animals referred to as botulism. The spores of Clostridium botulinum are found in soil and can grow in improperly sterilized and sealed food containers of home based canneries, which are the cause of many of the cases of botulism. The effects of botulism typically appear 18 to 36 hours after eating the foodstuffs infected with a Clostridium botulinum culture or spores. The botulinum toxin can apparently pass unattenuated through the lining of the gut and attack peripheral motor neurons. Symptoms of botulinum toxin intoxication can progress from difficulty walking, swallowing, and speaking to paralysis of the respiratory muscles and death.
Botulinum toxin type A is the most lethal natural biological agent known to man. About 50 picograms of a commercially available botulinum toxin type A (purified neurotoxin complex).sup.1 is a LD.sub.50 in mice (i.e. 1 unit). One unit of BOTOX.RTM. contains about 50 picograms (about 56 attomoles) of botulinum toxin type A complex. Interestingly, on a molar basis, botulinum toxin type A is about 1.8 billion times more lethal than diphtheria, about 600 million times more lethal than sodium cyanide, about 30 million times more lethal than cobra toxin and about 12 million times more lethal than cholera. Singh, Critical Aspects of Bacterial Protein Toxins, pages 63-84 (chapter 4) of Natural Toxins II, edited by B. R. Singh et al., Plenum Press, New York (1996) (where the stated LD.sub.50 of botulinum toxin type A of 0.3 ng equals 1 U is corrected for the fact that about 0.05 ng of BOTOX.RTM. equals 1 unit). One unit (U) of botulinum toxin is defined as the LD.sub.50 upon intraperitoneal injection into female Swiss Webster mice weighing 18 to 20 grams each.
 FNT .sup.1 Available from Allergan, Inc., of Irvine, Calif. under tradename BODX.RTM. in unit vials
Seven immunologically distinct botulinum neurotoxins have been characterized, these being respectively botulinum neurotoxin serotypes A, B, C.sub.1, D, E, F and G each of which is distinguished by neutralization with typespecific antibodies. The different serotypes of botulinum toxin vary in the animal species that they affect and in the severity and duration of the paralysis they evoke. For example, it has been determined that botulinum toxin type A is 500 times more potent, as measured by the rate of paralysis produced in the rat, than is botulinum toxin type B. Additionally, botulinum toxin type B has been determined to be non-toxic in primates at a dose of 480 U/kg which is about 12 times the primate LD.sub.50 for botulinum toxin type A. Botulinum toxin apparently binds with high affinity to cholinergic motor neurons, is translocated into the neuron and blocks the release of acetylcholine.
Regardless of serotype, the molecular mechanism of toxin intoxication appears to be similar and to involve at least three steps or stages. In the first step of the process, the toxin binds to the presynaptic membrane of the target neuron through a specific interaction between the heavy chain, H chain, and a cell surface receptor; the receptor is thought to be different for each type of botulinum toxin and for tetanus toxin. The carboxyl end segment of the H chain, H.sub.C, appears to be important for targeting of the toxin to the cell surface.
In the second step, the toxin crosses the plasma membrane of the poisoned cell. The toxin is first engulfed by the cell through receptor-mediated endocytosis, and an endosome containing the toxin is formed. The toxin then escapes the endosome into the cytoplasm of the cell. This step is thought to be mediated by the amino end segment of the H chain, H.sub.N, which triggers a conformational change of the toxin in response to a pH of about 5.5 or lower. Endosomes are known to possess a proton pump which decreases intra-endosomal pH. The conformational shift exposes hydrophobic residues in the toxin, which permits the toxin to embed itself in the endosomal membrane. The toxin (or at a minimum the light chain) then translocates through the endosomal membrane into the cytoplasm.
The last step of the mechanism of botulinum toxin activity appears to involve reduction of the disulfide bond joining the heavy chain, H chain, and the light chain, L chain. The entire toxic activity of botulinum and tetanus toxins is contained in the L chain of the holotoxin; the L chain is a zinc (Zn++) endopeptidase which selectively cleaves proteins essential for recognition and docking of neurotransmitter-containing vesicles with the cytoplasmic surface of the plasma membrane, and fusion of the vesicles with the plasma membrane. Tetanus neurotoxin, and botulinum toxins B, D, F, and G cause degradation of synaptobrevin (also called vesicle-associated membrane protein (VAMP)), a synaptosomal membrane protein. Most of the VAMP present at the cytoplasmic surface of the synaptic vesicle is removed as a result of any one of these cleavage events. Serotype A and E cleave SNAP-25. Serotype C.sub.1 was originally thought to cleave syntaxin, but was found to cleave syntaxin and SNAP-25. Each toxin specifically cleaves a different bond (except tetanus and type B which cleave the same bond).
Botulinum toxins have been used in clinical settings for the treatment of neuromuscular disorders characterized by hyperactive skeletal muscles. Botulinum toxin type A was approved by the U.S. Food and Drug Administration in 1989 for the treatment of blepharospasm, strabismus and hemifacial spasm. Non-type A botulinum toxin serotypes apparently have a lower potency and/or a shorter duration of activity as compared to botulinum toxin type A. Clinical effects of peripheral intramuscular botulinum toxin type A are usually seen within one week of injection. The typical duration of symptomatic relief from a single intramuscular injection of botulinum toxin type A averages about three months.
Although all the botulinum toxins serotypes apparently inhibit release of the neurotransmitter acetylcholine at the neuromuscular junction, they do so by affecting different neurosecretory proteins and/or cleaving these proteins at different sites. For example, botulinum types A and E both cleave the 25 kiloDalton (kD) synaptosomal associated protein (SNAP-25), but they target different amino acid sequences within this protein. Botulinum toxin types B, D, F and G act on vesicle-associated protein (VAMP, also called synaptobrevin), with each serotype cleaving the protein at a different site. Finally, botulinum toxin type C.sub.1 has been shown to cleave both syntaxin and SNAP-25. These differences in mechanism of action may affect the relative potency and/or duration of action of the various botulinum toxin serotypes. Apparently, a substrate for a botulinum toxin can be found in a variety of different cell types. See e.g. Biochem,J 1;339 (pt 1):159-65:1999, and Mov Disord, 10(3): 376:1995 (pancreatic islet B cells contain at least SNAP-25 and synaptobrevin).
The molecular weight of the botulinum toxin protein molecule, for all seven of the known botulinum toxin serotypes, is about 150 kD. Interestingly, the botulinum toxins are released by Clostridial bacterium as complexes comprising the 150 kD botulinum toxin protein molecule along with associated non-toxin proteins. Thus, the botulinum toxin type A complex can be produced by Clostridial bacterium as 900 kD, 500 kD and 300 kD forms. Botulinum toxin types B and C.sub.1 is apparently produced as only a 700 kD or 500 kD complex. Botulinum toxin type D is produced as both 300 kD and 500 kD complexes. Finally, botulinum toxin types E and F are produced as only approximately 300 kD complexes. The complexes (i.e. molecular weight greater than about 150 kD) are believed to contain a non-toxin hemaglutinin protein and a non-toxin and non-toxic nonhemaglutinin protein. These two non-toxin proteins (which along with the botulinum toxin molecule comprise the relevant neurotoxin complex) may act to provide stability against denaturation to the botulinum toxin molecule and protection against digestive acids when toxin is ingested. Additionally, it is possible that the larger (greater than about 150 kD molecular weight) botulinum toxin complexes may result in a slower rate of diffusion of the botulinum toxin away from a site of intramuscular injection of a botulinum toxin complex.
In vitro studies have indicated that botulinum toxin inhibits potassium cation induced release of both acetylcholine and norepinephrine from primary cell cultures of brainstem tissue. Additionally, it has been reported that botulinum toxin inhibits the evoked release of both glycine and glutamate in primary cultures of spinal cord neurons and that in brain synaptosome preparations botulinum toxin inhibits the release of each of the neurotransmitters acetylcholine, dopamine, norepinephrine (Habermann E., et al., Tetanus Toxin and Botulinum A and C Neurotoxins Inhibit Noradrenaline Release From Cultured Mouse Brain, J Neurochem 51(2);522-527:1988) CGRP, substance P and glutamate (Sanchez-Prieto, J., et al., Botulinum Toxin A Blocks Glutamate Exocytosis From Guinea Pig Cerebral Cortical Synaptosomes, Eur J. Biochem 165;675-681:1987. Thus, when adequate concentrations are used, stimulus-evoked release of most neurotransmitters is blocked by botulinum toxin. See e.g. Pearce, L. B., Pharmacologic Characterization of Botulinum Toxin For Basic Science and Medicine, Toxicon 35(9);1373-1412 at 1393 (1997); Bigalke H., et al., Botulinum A Neurotoxin Inhibits Non-Cholinergic Synaptic Transmission in Mouse Spinal Cord Neurons in Culture, Brain Research 360;318-324:1985; Habermann E., Inhibition by Tetanus and Botulinum A Toxin of the Release of [.sup.3 H]Noradrenaline and [.sup.3 H]GABA From Rat Brain Homogenate, Experientia 44;224-226:1988, Bigalke H., et al., Tetanus Toxin and Botulinum A Toxin Inhibit Release and Uptake of Various Transmitters, as Studied with Particulate Preparations From Rat Brain and Spinal Cord, Naunyn-Schmiedeberg's Arch Pharmacol 316;244-251:1981, and; Jankovic J. et al., Therapy With Botulinum Toxin, Marcel Dekker, Inc., (1994), page 5.
Botulinum toxin type A can be obtained by establishing and growing cultures of Clostridium botulinum in a fermenter and then harvesting and purifying the fermented mixture in accordance with known procedures. All the botulinum toxin serotypes are initially synthesized as inactive single chain proteins which must be cleaved or nicked by proteases to become neuroactive. The bacterial strains that make botulinum toxin serotypes A and G possess endogenous proteases and serotypes A and G can therefore be recovered from bacterial cultures in predominantly their active form. In contrast, botulinum toxin serotypes C.sub.1, D and E are synthesized by nonproteolytic strains and are therefore typically unactivated when recovered from culture. Serotypes B and F are produced by both proteolytic and nonproteolytic strains and therefore can be recovered in either the active or inactive form. However, even the proteolytic strains that produce, for example, the botulinum toxin type B serotype only cleave a portion of the toxin produced. The exact proportion of nicked to unnicked molecules depends on the length of incubation and the temperature of the culture. Therefore, a certain percentage of any preparation of, for example, the botulinum toxin type B toxin is likely to be inactive, possibly accounting for the known significantly lower potency of botulinum toxin type B as compared to botulinum toxin type A. The presence of inactive botulinum toxin molecules in a clinical preparation will contribute to the overall protein load of the preparation, which has been linked to increased antigenicity, without contributing to its clinical efficacy. Additionally, it is known that botulinum toxin type B has, upon intramuscular injection, a shorter duration of activity and is also less potent than botulinum toxin type A at the same dose level.
High quality crystalline botulinum toxin type A can be produced from the Hall A strain of Clostridium botulinum with characteristics of .gtoreq.3.times.10.sup.7 U/mg, an A.sub.260 /A.sub.278 of less than 0.60 and a distinct pattern of banding on gel electrophoresis. The known Shantz process can be used to obtain crystalline botulinum toxin type A, as set forth in Shantz, E. J., et al, Properties and Use of Botulinum Toxin and Other Microbial Neurotoxins in Medicine, Microbiol Rev. 56;80-99:1992. Generally, the botulinum toxin type A complex can be isolated and purified from an anaerobic fermentation by cultivating Clostridium botulinum type A in a suitable medium. The known process can also be used, upon separation out of the non-toxin proteins, to obtain pure botulinum toxins, such as for example: purified botulinum toxin type A with an approximately 150 kD molecular weight with a specific potency of 1-2.times.10.sup.8 LD.sub.50 U/mg or greater; purified botulinum toxin type B with an approximately 156 kD molecular weight with a specific potency of 1-2.times.10.sup.8 LD.sub.50 U/mg or greater, and; purified botulinum toxin type F with an approximately 155 kD molecular weight with a specific potency of 1-2.times.10.sup.7 LD.sub.50 U/mg or greater.
Botulinum toxins and/or botulinum toxin complexes can be obtained from List Biological Laboratories, Inc., Campbell, Calif.; the Centre for Applied Microbiology and Research, Porton Down, U.K.; Wako (Osaka, Japan), Metabiologics (Madison, Wis.) as well as from Sigma Chemicals of St. Louis, Mo.
Pure botulinum toxin is so labile that it is generally not used to prepare a pharmaceutical composition. Furthermore, the botulinum toxin complexes, such as the toxin type A complex are also extremely susceptible to denaturation due to surface denaturation, heat, and alkaline conditions. Inactivated toxin forms toxoid proteins which may be immunogenic. The resulting antibodies can render a patient refractory to toxin injection.
As with enzymes generally, the biological activities of the botulinum toxins (which are intracellular peptidases) are dependent, at least in part, upon their three dimensional conformation. Thus, botulinum toxin type A is detoxified by heat, various chemicals surface stretching and surface drying. Additionally, it is known that dilution of the toxin complex obtained by the known culturing, fermentation and purification to the much, much lower toxin concentrations used for pharmaceutical composition formulation results in rapid detoxification of the toxin unless a suitable stabilizing agent is present. Dilution of the toxin from milligram quantities to a solution containing nanograms per milliliter presents significant difficulties because of the rapid loss of specific toxicity upon such great dilution. Additionally, the toxin may be used months or years after the toxin containing pharmaceutical composition is formulated. Significantly, it is known that the toxin can be stabilized during the manufacture and compounding processes as well as during storage by use of a stabilizing agent such as albumin and gelatin.
A commercially available botulinum toxin containing pharmaceutical composition is sold under the trademark BOTOX.RTM. (available from Allergan, Inc., of Irvine, Calif.). BOTOX.RTM. consists of a purified botulinum toxin type A complex, albumin and sodium chloride packaged in sterile, vacuum-dried from. The botulinum toxin type A is made from a culture of the Hall strain of Clostridium botulinum grown in a medium containing N-Z amine and yeast extract. The botulinum toxin type A complex is purified from the culture solution by a series of acid precipitations to a crystalline complex consisting of the active high molecular weight toxin protein and an associated hemagglutinin protein. The crystalline complex is re-dissolved in a solution containing saline and albumin and sterile filtered (0.2 microns) prior to vacuum-drying. The vacuum-dried product is stored in a freezer at or below -5.degree. C. BOTOX.RTM. can be reconstituted with sterile, non-preserved saline prior to intramuscular injection. Each vial of BOTOX.RTM. contains about 100 units (U) of Clostridium botulinum toxin type A purified neurotoxin complex, 0.5 milligrams of human serum albumin and 0.9 milligrams of sodium chloride in a sterile, vacuum-dried form without a preservative.
To reconstitute vacuum-dried BOTOX.RTM., sterile normal saline without a preservative (0.9% Sodium Chloride Injection) is used by drawing up the proper amount of diluent in the appropriate size syringe. Since BOTOX.RTM. may be denatured by bubbling or similar violent agitation, the diluent is gently injected into the vial. For sterility reasons BOTOX.RTM. is preferably administered within four hours after the vial is removed from the freezer and reconstituted. During these four hours, reconstituted BOTOX.RTM. can be stored in a refrigerator at about 2.degree. C. to about 8.degree. C. Reconstituted, refrigerated BOTOX.RTM. retains its potency for at least two weeks. Neurology, 48:249-53:1997.
It has been reported that botulinum toxin type A has been used in clinical settings as follows:
(1) about 75-125 units of BOTOX.RTM. per intramuscular injection (multiple muscles) to treat cervical dystonia; PA1 (2) 5-10 units of BOTOX.RTM. per intramuscular injection to treat glabellar lines (brow furrows) (5 units injected intramuscularly into the procerus muscle and 10 units injected intramuscularly into each corrugator supercilii muscle); PA1 (3) about 30-80 units of BOTOX.RTM. to treat constipation by intrasphincter injection of the puborectalis muscle; PA1 (4) about 1-5 units per muscle of intramuscularly injected BOTOX.RTM. to treat blepharospasm by injecting the lateral pre-tarsal orbicularis oculi muscle of the upper lid and the lateral pre-tarsal orbicularis oculi of the lower lid. PA1 (5) to treat strabismus, extraocular muscles have been injected intramuscularly with between about 1-5 units of BOTOX.RTM., the amount injected varying based upon both the size of the muscle to be injected and the extent of muscle paralysis desired (i.e. amount of diopter correction desired). PA1 (6) to treat upper limb spasticity following stroke by intramuscular injections of BOTOX.RTM. into five different upper limb flexor muscles, as follows: PA1 (7) to treat migraine, pericranial injected (injected symmetrically into glabellar, frontalis and temporalis muscles) injection of 25 U of BOTOX.RTM. has showed significant benefit as a prophylactic treatment of migraine compared to vehicle as measured by decreased measures of migraine frequency, maximal severity, associated vomiting and acute medication use over the three month period following the 25 U injection.
(a) flexor digitorum profundus: 7.5 U to 30 U PA2 (b) flexor digitorum sublimus: 7.5 U to 30 U PA2 (c) flexor carpi ulnaris: 10 U to 40 U PA2 (d) flexor carpi radialis: 15 U to 60 U PA2 (e) biceps brachii: 50 U to 200 U. Each of the five indicated muscles has been injected at the same treatment session, so that the patient receives from 90 U to 360 U of upper limb flexor muscle BOTOX.RTM. by intramuscular injection at each treatment session.
It is known that botulinum toxin type A can have an efficacy for up to 12 months (European J. Neurology 6 (Supp 4): S111-S1150:1999), and in some circumstances for as long as 27 months, (The Laryngoscope 109: 1344-1346:1999). However, the usual duration of an intramuscular injection of Botox.RTM. is typically about 3 to 4 months.
The success of botulinum toxin type A to treat a variety of clinical conditions has led to interest in other botulinum toxin serotypes. A study of two commercially available botulinum type A preparations (BOTOX.RTM. and Dysport.RTM.) and preparations of botulinum toxins type B and F (both obtained from Wako Chemicals, Japan) has been carried out to determine local muscle weakening efficacy, safety and antigenic potential. Botulinum toxin preparations were injected into the head of the right gastrocnemius muscle (0.5 to 200.0 units/kg) and muscle weakness was assessed using the mouse digit abduction scoring assay (DAS). ED.sub.50 values were calculated from dose response curves. Additional mice were given intramuscular injections to determine LD.sub.50 doses. The therapeutic index was calculated as LD.sub.50 /ED.sub.50. Separate groups of mice received hind limb injections of BOTOX.RTM. (5.0 to 10.0 units/kg) or botulinum toxin type B (50.0 to 400.0 units/kg), and were tested for muscle weakness and increased water consumption, the later being a putative model for dry mouth. Antigenic potential was assessed by monthly intramuscular injections in rabbits (1.5 or 6.5 ng/kg for botulinum toxin type B or 0.15 ng/kg for BOTOX.RTM.). Peak muscle weakness and duration were dose related for all serotypes. DAS ED.sub.50 values (units/kg) were as follows: BOTOX.RTM.: 6.7, Dysport.RTM.: 24.7, botulinum toxin type B: 27.0 to 244.0, botulinum toxin type F: 4.3. BOTOX.RTM. had a longer duration of action than botulinum toxin type B or botulinum toxin type F. Therapeutic index values were as follows: BOTOX.RTM.: 10.5, Dysport.RTM.: 6.3, botulinum toxin type B: 3.2. Water consumption was greater in mice injected with botulinum toxin type B than with BOTOX.RTM., although botulinum toxin type B was less effective at weakening muscles. After four months of injections 2 of 4 (where treated with 1.5 ng/kg) and 4 of 4 (where treated with 6.5 ng/kg) rabbits developed antibodies against botulinum toxin type B. In a separate study, 0 of 9 BOTOX.RTM. treated rabbits demonstrated antibodies against botulinum toxin type A. DAS results indicate relative peak potencies of botulinum toxin type A being equal to botulinum toxin type F, and botulinum toxin type F being greater than botulinum toxin type B. With regard to duration of effect, botulinum toxin type A was greater than botulinum toxin type B, and botulinum toxin type B duration of effect was greater than botulinum toxin type F. As shown by the therapeutic index values, the two commercial preparations of botulinum toxin type A (BOTOX.RTM. and Dysport.RTM.) are different. The increased water consumption behavior observed following hind limb injection of botulinum toxin type B indicates that clinically significant amounts of this serotype entered the murine systemic circulation. The results also indicate that in order to achieve efficacy comparable to botulinum toxin type A, it is necessary to increase doses of the other serotypes examined. Increased dosage can comprise safety. Furthermore, in rabbits, type B was more antigenic than was BOTOX.RTM., possibly because of the higher protein load injected to achieve an effective dose of botulinum toxin type B. Eur J Neurol Nov. 6, 1999 (Suppl 4):S3-S10.
In addition to having pharmacologic actions at a peripheral location, a botulinum toxin can also exhibit a denervation effect in the central nervous system. Wiegand et al, Naunyn-Schmiedeberg's Arch. Pharmacol. 1976; 292, 161-165, and Habermann, Naunyn-Schmiedeberg's Arch. Pharmacol. 1974; 281, 47-56 reported that botulinum toxin is able to ascend to the spinal area by retrograde transport. As such, a botulinum toxin injected at a peripheral location, for example intramuscularly, can potentially be retrograde transported to the spinal cord.
U.S. Pat. No. 5,989,545 discloses that a modified clostridial neurotoxin or fragment thereof, preferably a botulinum toxin, chemically conjugated or recombinantly fused to a particular targeting moiety can be used to treat pain by administration of the agent to the spinal cord.
Acetylcholine
Typically only a single type of small molecule neurotransmitter is released by each type of neuron in the mammalian nervous system. The neurotransmitter acetylcholine is secreted by neurons in many areas of the brain, but specifically by the large pyramidal cells of the motor cortex, by several different neurons in the basal ganglia, by the motor neurons that innervate the skeletal muscles, by the preganglionic neurons of the autonomic nervous system (both sympathetic and parasympathetic), by the postganglionic neurons of the parasympathetic nervous system, and by some of the postganglionic neurons of the sympathetic nervous system. Essentially, only the postganglionic sympathetic nerve fibers to the sweat glands, the piloerector muscles and a few blood vessels are cholinergic as most of the postganglionic neurons of the sympathetic nervous system secret the neurotransmitter norepinephine. In most instances acetylcholine has an excitatory effect. However, acetylcholine is known to have inhibitory effects at some of the peripheral parasympathetic nerve endings, such as inhibition of heart rate by the vagal nerve.
The efferent signals of the autonomic nervous system are transmitted to the body through either the sympathetic nervous system or the parasympathetic nervous system. The preganglionic neurons of the sympathetic nervous system extend from preganglionic sympathetic neuron cell bodies located in the intermediolateral horn of the spinal cord. The preganglionic sympathetic nerve fibers, extending from the cell body, synapse with postganglionic neurons located in either a paravertebral sympathetic ganglion or in a prevertebral ganglion. Since the preganglionic neurons of both the sympathetic and parasympathetic nervous system are cholinergic, application of acetylcholine to the ganglia will excite both sympathetic and parasympathetic postganglionic neurons.
Acetylcholine activates two types of receptors, muscarinic and nicotinic receptors. The muscarinic receptors are found in all effector cells stimulated by the postganglionic, neurons of the parasympathetic nervous system as well as in those stimulated by the postganglionic cholinergic neurons of the sympathetic nervous system. The nicotinic receptors are found in the adrenal medulla, as well as within the autonomic ganglia, that is on the cell surface of the postganglionic neuron at the synapse between the preganglionic and postganglionic neurons of both the sympathetic and parasympathetic systems. Nicotinic receptors are also found in many nonautonomic nerve endings, for example in the membranes of skeletal muscle fibers at the neuromuscular junction.
Acetylcholine is released from cholinergic neurons when small, clear, intracellular vesicles fuse with the presynaptic neuronal cell membrane. A wide variety of non-neuronal secretory cells, such as, adrenal medulla (as well as the PC12 cell line) and pancreatic islet cells release catecholamines and parathyroid hormone, respectively, from large dense-core vesicles. The PC12 cell line is a clone of rat pheochromocytoma cells extensively used as a tissue culture model for studies of sympathoadrenal development. Botulinum toxin inhibits the release of both types of compounds from both types of cells in vitro, permeabilized (as by electroporation) or by direct injection of the toxin into the denervated cell. Botulinum toxin is also known to block release of the neurotransmitter glutamate from cortical synaptosomes cell cultures.
A neuromuscular junction is formed in skeletal muscle by the proximity of axons to muscle cells. A signal transmitted through the nervous system results in an action potential at the terminal axon, with activation of ion channels and resulting release of the neurotransmitter acetylcholine from intraneuronal synaptic vesicles, for example at the motor endplate of the neuromuscular junction. The acetylcholine crosses the extracellular space to bind with acetylcholine receptor proteins on the surface of the muscle end plate. Once sufficient binding has occurred, an action potential of the muscle cell causes specific membrane ion channel changes, resulting in muscle cell contraction. The acetylcholine is then released from the muscle cells and metabolized by cholinesterases in the extracellular space. The metabolites are recycled back into the terminal axon for reprocessing into further acetylcholine.
Therefore, a need exists for a biocompatible, nonimmunogenic, nonbiodegradable implant which permits long term continuous release of a therapeutically effective neurotoxin in a human patient.